It is mechanical energy because when it reaches a height it gets potential energy and as it is continuously moving it has kinetic energy and the sum of these both is mechanical energy.

Introduction:
During the motion of a pendulum in a clock, it acquires various forms of energy. Understanding the different types of energy involved can help explain the mechanism behind the pendulum's motion and how it keeps time.

Types of Energy:

1. Potential Energy:
- At the highest point of its swing, the pendulum has the maximum potential energy.
- Potential energy is stored energy that an object possesses due to its position or condition.
- In a pendulum, the potential energy is highest when the pendulum bob is at its maximum height, pulled away from its equilibrium position.

2. Kinetic Energy:
- As the pendulum swings down from its highest point, the potential energy is converted into kinetic energy.
- Kinetic energy is the energy possessed by an object due to its motion.
- At the lowest point of the swing, the pendulum has the maximum kinetic energy.

3. Gravitational Potential Energy:
- The pendulum bob is raised to its highest point by doing work against gravity.
- This work is stored as gravitational potential energy.
- When the bob is released, the potential energy is converted into kinetic energy as it falls.

4. Mechanical Energy:
- Mechanical energy is the sum of potential energy and kinetic energy in a system.
- In the case of a pendulum, mechanical energy is conserved as the potential energy is converted into kinetic energy and vice versa.

Conclusion:
During the motion of a pendulum in a clock, it acquires potential energy at the highest point, which is then converted into kinetic energy as it swings down. The work done against gravity to raise the pendulum bob results in gravitational potential energy. The overall mechanical energy of the system remains constant, with the interconversion of potential and kinetic energy. This energy transfer is crucial for the oscillatory motion of the pendulum, enabling it to keep time in a clock.